When a polyol with a fast-acting catalyst is mixed with isocyanate in a mixing block and the mixed foamable liquid is dispensed at spaced points on a traveling substrate which forms a facing on the finished foam board, foam tends to build up on the inner walls of the conduits by which the foamable liquiud is distributed from the mixing block to the substrate. The fast-acting catalyst is dictated by the requirement of economy in production. In actual practice, before my invention, the build-up of foam caused blockages in twenty to thirty minutes, requiring shut-down of the production line for cleaning of the conduits.